Stator for an electric motor and a method of manufacturing a stator

ABSTRACT

A stator for an electric motor, the stator comprising a back-iron having a first surface and a heat sink having a second surface, wherein the first surface of the back-iron includes a plurality of keying features arranged to retain the heat sink to the back-iron, wherein the heat sink is arranged to provide cooling to the back-iron via the second surface, and wherein the second surface has complementary keying features to the plurality of keying features on the first surface that are formed by casting the heat sink on the first surface of the back-iron.

The present invention relates to a stator for an electric motor and amethod of manufacturing a stator for an electric motor.

With increased interest being placed in environmentally friendlyvehicles there has been a corresponding increase in interest in the useof electric motors for providing drive torque for electric vehicles.

Electric motors work on the principle that a current carrying wire willexperience a force when in the presence of a magnetic field. When thecurrent carrying wire is placed perpendicular to the magnetic field theforce on the current carrying wire is proportional to the flux densityof the magnetic field. Typically, in an electric motor the force on acurrent carrying wire is formed as a rotational torque.

Examples of known types of electric motor include the induction motor,brushless permanent magnet motor, switched reluctance motor andsynchronous slip ring motor, which have a rotor and a stator, as is wellknown to a person skilled in the art.

In the commercial arena three phase electric motors are the most commonkind of electric motor available.

A three phase electric motor typically includes three coil sets, whereeach coil set is arranged to generate a magnetic field associated withone of the three phases of an alternating voltage.

To increase the number of magnetic poles formed within an electricmotor, each coil set will typically have a number of coil sub-sets thatare distributed around the periphery of the electric motor, which aredriven to produce a rotating magnetic field.

The three coil sets of a three phase electric motor are typicallyconfigured in either a delta or wye configuration. The current flowthrough the coil windings will generate heat, which can result in areduction in efficiency and power generating capabilities of an electricmotor, where effective cooling is essential for high continuous torque.

The current flow through the coil windings is typically controlled via acontrol unit, where for a three phase electric motor having a DC powersupply, the control unit will typically include a three phase bridgeinverter that generates a three phase voltage supply for driving theelectric motor. Each of the respective voltage phases is applied to arespective coil set of the electric motor.

Typically, the three phase bridge inverter will generate a three phasevoltage supply using a form of pulse width modulation (PWM) voltagecontrol. PWM control works by using the motor inductance to average outan applied pulse voltage to drive the required current into the motorcoils. Using PWM control an applied voltage is switched across the motorcoils. During this on period, the current rises in the motor coils at arate dictated by its inductance and the applied voltage. The PWM controlis then required to switch off before the current has changed too muchso that precise control of the current is achieved.

A three phase bridge inverter includes a number of switching devices,for example power electronic switches such as Insulated Gate BipolarTransistor (IGBT) switches.

In the context of an electric vehicle motor, a drive design that isbecoming increasing popular is an integrated in-wheel electric motordesign in which an electric motor and its associated control system areintegrated within a wheel of a vehicle.

However, the integration of an electric motor, and its associatedcontrol system, within a wheel of a vehicle can impose increased thermalmanagement considerations upon the electric motor, which canadditionally result in a reduction in efficiency and power generatingcapabilities of an electric motor.

In accordance with an aspect of the present invention there is provideda stator and a method of manufacturing a stator according to theaccompanying claims.

The invention as claimed provides the advantage of improving theretention of a stator back-iron to a heat sink, thereby improving thestructural integrity of the electric motor, while also increasing thesurface area of the interface between the stator back-iron and the heatsink to allow the thermal interface between these two components to beimproved, thereby improving heat transfer from the stator back-iron tothe heat sink by improving the thermal conductivity across the interfacebetween the back-iron and the heat sink.

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 illustrates an exploded view of a motor embodying the presentinvention;

FIG. 2 is an exploded view of the motor of FIG. 1 from an alternativeangle;

FIG. 3 illustrates a cross sectional view of a stator;

FIG. 4 illustrates a cross sectional view of a stator;

FIG. 5 illustrates a cooling channel of a stator according to anembodiment of the present invention;

FIG. 6 illustrates a cooling channel of a stator according to anembodiment of the present invention;

FIG. 7 illustrates a back iron lamination according to an embodiment ofthe present invention;

FIG. 8 illustrates a laminated back iron according to an embodiment ofthe present invention.

FIG. 1 and FIG. 2 illustrate an electric motor assembly incorporating anelectric motor having a stator according to the present invention wherethe electric motor assembly includes built in electronics and isconfigured for use as a hub motor or in-wheel electric motor built to beaccommodated within a wheel. However, the present invention could beincorporated in any form of electric motor. The electric motor can alsobe configured as a generator.

For the purposes of the present embodiment, as illustrated in FIG. 1 andFIG. 2, the in-wheel electric motor includes a stator assembly 252 and arotor assembly 240. The stator assembly 252 comprising a heat sink 253having a cooling channel, a laminated back-iron (not shown), multiplecoils 254, an electronics module 255 mounted in a rear portion of thestator for driving the coils, and a capacitor (not shown) mounted on thestator within a recess formed on the rear portion of the stator. In apreferred embodiment the capacitor is an annular capacitor element.

Stator tooth laminations are formed on the laminated back-iron, with thecoils 254 being formed on the stator tooth laminations to form coilwindings. A stator cover 256 is mounted on the rear portion of thestator 252, enclosing the electronics module 255 to form the statorassembly 252, which may then be fixed to a vehicle and does not rotaterelative to the vehicle during use.

The electronics module 255 includes two control devices 400, where eachcontrol device 400 includes an inverter and control logic, which in thepresent embodiment includes a processor, for controlling the operationof the inverter.

To reduce the effects of inductance on the inverters, housed in theelectronics module 255, when switching current, the capacitors mountedon the stator is used as a local voltage source for the electric motorinverters. By placing a capacitor close to an inverter the inductanceassociated with the voltage source is minimised.

A rotor 240 comprises a front portion 220 and a cylindrical portion 221forming a cover, which substantially surrounds the stator assembly 252.The rotor includes a plurality of permanent magnets 242 arranged aroundthe inside of the cylindrical portion 221. For the purposes of thepresent embodiment 32 magnet pairs are mounted on the inside of thecylindrical portion 221. However, any number of magnet pairs may beused.

The magnets are in close proximity to the coil windings on the stator252 so that magnetic fields generated by the coils interact with themagnets 242 arranged around the inside of the cylindrical portion 221 ofthe rotor assembly 240 to cause the rotor assembly 240 to rotate. As thepermanent magnets 242 are utilized to generate a drive torque fordriving the electric motor, the permanent magnets are typically calleddrive magnets.

The rotor 240 is attached to the stator 252 by a bearing block 223. Thebearing block 223 can be a standard bearing block as would be used in avehicle to which this motor assembly is to be fitted. The bearing blockcomprises two parts, a first part fixed to the stator and a second partfixed to the rotor. The bearing block is fixed to a central portion 253of the wall of the stator 252 and also to a central portion 225 of thehousing wall 220 of the rotor 240. The rotor 240 is thus rotationallyfixed to the vehicle with which it is to be used via the bearing block223 at the central portion 225 of the rotor 240. This has an advantagein that a wheel rim and tyre can then be fixed to the rotor 240 at thecentral portion 225 using the normal wheel bolts to fix the wheel rim tothe central portion of the rotor and consequently firmly onto therotatable side of the bearing block 223. The wheel bolts may be fittedthrough the central portion 225 of the rotor through into the bearingblock itself. With both the rotor 240 and the wheel being mounted to thebearing block 223 there is a one to one correspondence between the angleof rotation of the rotor and the wheel.

FIG. 2 shows an exploded view of the same assembly as FIG. 1 from theopposite side showing the stator assembly 252 and rotor assembly 240.The rotor assembly 240 comprises the outer rotor wall 220 andcircumferential wall 221 within which magnets 242 are circumferentiallyarranged. As previously described, the stator assembly 252 is connectedto the rotor assembly 240 via the bearing block at the central portionsof the rotor and stator walls.

A V shaped seal is provided between the circumferential wall 221 of therotor and the outer edge of the stator.

The rotor also includes a set of magnets 227 for position sensing,otherwise known as commutation magnets, which in conjunction withsensors mounted on the stator allows for a rotor flux angle to beestimated. The rotor flux angle defines the positional relationship ofthe drive magnets to the coil windings. Alternatively, in place of a setof separate magnets the rotor may include a ring of magnetic materialthat has multiple poles that act as a set of separate magnets.

To allow the commutation magnets to be used to calculate a rotor fluxangle, preferably each drive magnet has an associated commutationmagnet, where the rotor flux angle is derived from the flux angleassociated with the set of commutation magnets by calibrating themeasured commutation magnet flux angle. To simplify the correlationbetween the commutation magnet flux angle and the rotor flux angle,preferably the set of commutation magnets has the same number of magnetsor magnet pole pairs as the set of drive magnet pairs, where thecommutation magnets and associated drive magnets are approximatelyradially aligned with each other. Accordingly, for the purposes of thepresent embodiment the set of commutation magnets has 32 magnet pairs,where each magnet pair is approximately radially aligned with arespective drive magnet pair.

A sensor, which in this embodiment is a Hall sensor, is mounted on thestator. The sensor is positioned so that as the rotor rotates each ofthe commutation magnets that form the commutation magnet ringrespectively rotates past the sensor.

As the rotor rotates relative to the stator the commutation magnetscorrespondingly rotate past the sensor with the Hall sensor outputtingan AC voltage signal, where the sensor outputs a complete voltage cycleof 360 electrical degrees for each magnet pair that passes the sensor.

For improved position detection, preferably the sensor includes anassociated second sensor placed 90 electrical degrees displaced from thefirst sensor.

The motor in this embodiment includes two coil sets with each coil sethaving three coil sub-sets that are coupled in a wye configuration toform a three phase sub-motor, resulting in the motor having two threephase sub-motors. However, although the present embodiment describes anelectric motor having two coil sets (i.e. two sub motors) the motor mayequally have one or more coil sets with associated control devices (i.e.the electric motor can be configured as a single motor or a plurality ofsub motors). For example in a preferred embodiment the motor includeseight coil sets with each coil set 60 having three coil sub-sets thatare coupled in a wye configuration to form a three phase sub-motor,resulting in the motor having eight three phase sub-motors.

FIGS. 3 to 6 illustrates a preferred embodiment of the cooling aspectsof the stator heat sink 253, for providing cooling to a plurality ofdifferent electric motor components. However, the stator heat sink cantake any form that allows the provision of cooling to the laminated backiron.

FIG. 3 illustrates a cross sectional view of the stator heat sink 253,where a cooling channel 300 is formed in a circumferential portion ofthe stator heat sink 253. Coolant is arranged to flow around the coolingchannel 300, as described below.

The cooling channel 300 is arranged to have a first portion 310 that isorientated to be substantially perpendicular to a second portion 320 ofthe cooling channel 300. The first portion 310 and second portion 320form a single cooling channel 300, where coolant is arranged to flowthrough the first portion 310 and the second portion 320. Additionally,the first portion 310 and second portion 320 are coupled to allowcoolant to flow between the first portion 310 and second portion 320.

As illustrated in FIG. 4, one side of the first portion 310 of thecooling channel 300 is arranged to extend adjacent to a first surface410 of the stator heat sink 253.

A second side of the first portion 310 of the cooling channel 300 isarranged to extend adjacent to a second surface 420 of the stator heatsink 253 upon which the control devices 400 are arranged to be mounted,as illustrated in FIG. 4.

Additionally, one side of the second portion 320 of the cooling channel300 is arranged to extend adjacent to a third surface 430 of the statorheat sink 253. The stator teeth and coil windings 254 are arranged to bemounted on the third surface 430, as illustrated in FIG. 4.

A second side of the second portion 320 of the cooling channel 300 isarranged to extend adjacent to a fourth surface 440 of the stator heatsink 253, where the fourth surface 440 forms one side of the recess 257for housing the capacitor 450, as illustrated in FIG. 4.

Accordingly, the preferred embodiment of the heat sink 253 provides acooling arrangement having a cooling channel arranged to cool theelectrical coils 254, a first electrical device (i.e. the controldevices 400) and a second electrical device (i.e. the capacitor 450).

To illustrate a preferred configuration for the cooling channel 300,FIG. 5 represents a preferred embodiment of a cooling channel moulding500 used to form a cooling channel 300 within a stator heat sink 253.Preferably, the stator heat sink 257 is formed using a sand castingprocess, where the cooling channel moulding 300 is formed from sand andcast onto the laminated back iron, as described below. However, anysuitable form of casting may be used.

As would be appreciated by a person skilled in the art, the solidsections of the cooling channel moulding 500 correspond to the sectionsof the cooling channel within which coolant is able to flow within thestator heat sink 253.

Correspondingly, the gaps formed within the cooling channel moulding 500correspond to solid sections within the cooling channel 300 in whichcoolant is unable to flow.

Feature A on the cooling channel moulding 500 corresponds to the coolingchannel inlet, where cooling fluid is input into the cooling channel.Within the stator heat sink 253, the coolant travels around theorthogonal oriented sections of the cooling channel 300, whichcorresponds to the first portion 310 and second portion 320 of thecooling channel 300, in a clockwise direction based on the orientationof the cooling channel moulding illustrated in FIG. 5.

Feature B on the cooling channel moulding 500 corresponds to the coolingchannel outlet, where cooling fluid is output from the cooling channel300.

The dimensions of the inlet section of the cooling channel 300 arepreferably configured to determine the amount of cooling fluid that isdirected to circulate in the respective first portion 310 and secondportion 320 of the cooling channel 300. For example, protrusions withinthe cooling channel inlet may be used to set the required percentage ofcooling fluid to be circulated within the respective first portion 310and second portion 320 of the cooling channel 300. By way ofillustration, if a greater amount of cooling is required in the secondportion of the cooling channel, that is to say the cooling channelportion sandwiched between the capacitor 450 and the bottom of thelaminated back iron, protrusions within the cooling channel inlet can beused to send the majority of the coolant along this portion of thecooling channel 300.

The amount of coolant that flows between the first portion 310 and thesecond portion 320 of the coolant channel 300 is minimised by includinga number of protrusions 510 at the interface between the first portion310 and the second portion 320 of the cooling channel 300, asillustrated in FIG. 5. The protrusions 510 act as an obstacle to thecoolant flow between the first portion 310 and second portion 320 of thecoolant channel 300 and accordingly inhibit the movement of coolantfluid between the two portions 310, 320, thereby allowing a singlecoolant channel to be used but where the amount of coolant in theseparate portions 310, 320 can be independently controlled.

Preferably the effectiveness of the protrusions 510 may be enhanced byextending them to form a rib 520, as illustrated in FIG. 6, to covermore substantial portions of the cooling channel.

As the flow of coolant in the first portion 310 and the second portion320 of the coolant channel 300 are effectively independent the crosssectional areas of the first portion 310 and second portion 320 of thecooling channel 300 can be independently selected depending upon thecooling needs associated with the different cooling channel portions.For example, if increased cooling is required the cross sectional areacan be reduced to increase fluid flow. However, preferably for thepresent embodiment fluid flow is kept below five meters per second tominimise the risk of erosion of the stator heat sink 253.

To further fine tune the cooling characteristics of the first portion310 and second portion 320 of the cooling channel 300 the crosssectional areas of different sections within the first portion 310and/or second portion 320 can be changed depending upon specific coolingrequirements. For example, as each of the two control device 400 mountedon the stator heat sink 253 only extend around a quarter of the statorheat sink circumference, to enhance cooling over the sections of thestator heat sink 253 upon which are mounted the control devices 400, thecross sectional area of the first portion 310 of the cooling channel 300at these locations can be reduced relative to the sections of the firstportion 310 at which the control devices 400 are not mounted, therebyincreasing the cooling capability of the stator heat sink 253 at thepositions where the control device 400 are mounted.

Accordingly, based on the cooling requirements it is possible to notonly vary the amount of cooling between the first portion 310 and thesecond portion 320 of the cooling channel 300 but to also to vary theamount of cooling around both the first portion 310 and the secondportion 320 of the cooling channel 300.

A further method of increasing the amount of cooling provided by thestator heat sink 253 is to increase the surface area of the first and/orsecond portion of the cooling channel. One method to provide increasedsurface area at targeted locations, for example under the controldevices 400, would be to incorporated cooling ribs 530, as illustratedin FIG. 6.

Although the present embodiments, describe a single cooling channelhaving orthogonally oriented portions that are configured to provideoptimum cooling to separate elements of the electric motor andassociated control system, the electric motor may also includeadditional cooling channels for cooling other components, where theadditional cooling channels may be of conventional design or asdescribed above.

FIG. 7 illustrates a view of the laminated back-iron 700, where thelaminated back-iron 700 is constructed from a plurality of individuallaminations 710, where an individual lamination 710 is illustrated inFIG. 8, as is well known to a person skilled in the art.

Individual laminations may be joined together by any suitable means, forexample by welding, adhesive bonding and mechanical joints such ascleating and interlocking.

Preferably the laminations 710 are made from electrical steel, typicallyhaving a small hysteresis area and high permeability.

Each lamination 710 has a plurality of inner protrusions 720 located onthe inner radial surface of the lamination that extend radially inwardsfrom the inner radial surface. The inner protrusions 720 act as a keyingfeature when interfaced with the stator heat sink 253, as describedbelow. Keying features are features that allow for an increase insurface area of a surface that preferably provide for an increase inmechanical adhesion to a feature that is cast onto the keying feature.

Although the size and position of the inner protrusions may be randomlypositioned and sized on the inner surface, preferably, and for thepurposes of the present embodiment the inner protrusions 720 are equallyspaced with respect to adjacent inner protrusions 720 and havesubstantially the same dimensions, as illustrated in FIG. 8. Typically,the size and position of the protrusions would be determined by factorssuch as the number and thickness of laminations, the number of teeth,the diameter of the back iron and the thickness of the stator wall.Preferably, to minimise the impact of the protrusions on the magneticcircuit of the electric motor, the protrusions are arranged to have thesame symmetry as the stator teeth, with the protrusions positionedmidway between each of the stator teeth.

As illustrated in FIG. 7 and FIG. 8, each lamination 710 additionallyhas a plurality of outer protrusions 730 located on the outer radialsurface of the lamination 710. As illustrated, the outer protrusions 730are equally spaced with respect to adjacent outer protrusions 730 andhave substantially the same dimensions. When the individual laminations710 are combined to form the laminated back iron 700, the outerprotrusions 730 on the individual laminations 710 are oriented withrespect to each other to form corresponding protrusions that extendthree dimensionally in an axial direction on the laminated back iron700, where the outer protrusions formed on the laminated back iron 700are arranged to receive a stator tooth (not shown), for example, asdescribed in patents GB2477520 and GB2507072. The inner protrusions 720on the individual laminations 710 are combined to form a keying patternon the inner surface of the laminated back iron 700.

Although the present embodiment describes a stator back iron 700 havinga plurality of outer protrusions 730, where each protrusion is arrangedto receive a stator tooth, equally, the outer protrusions may beconfigured to form stator teeth that are integral to the stator backiron 700.

Although the inner protrusions 720 on the individual laminations 710 canbe oriented with respect to each other to form corresponding protrusionsthat extend three dimensionally in an axial direction on the laminatedback iron 700, preferably to provide different keying structures forimproved retention of the stator heat sink 253 to the laminated backiron 700, the inner protrusions 720 on the individual laminations 710are oriented at different angles with respect to each other to formdifferent three dimensional keying structures on the inner surface ofthe laminated back iron 700, as illustrated in FIG. 8. Accordingly, in apreferred embodiment, all or some of the plurality of laminations 710that form the back iron 700 are positioned at rotationally differentangles with respect to each other, thereby forming keying features atdifferent rotational positions along the axis of the back iron 700.

Accordingly, all the inner protrusions 720 on the individual laminations710 can be oriented the same with respect to each other to formcorresponding protrusions that extend three dimensionally in an axialdirection on the laminated back iron 700 or all the inner protrusions720 on the individual laminations 710 can be oriented differently withrespect to each other to form a complex keying structure, or acombination of some of the individual laminations 710 can be orientedthe same with respect to each other to form corresponding protrusionsthat extend three dimensionally in an axial direction on the laminatedback iron 700 for a given distance then a different orientation is usedfor the next set of laminations for a given distance to create a keyingstructure that is in a different orientation to the initial keyingstructure.

For example, the keying structure on the inner axial portion of theinner surface of the laminated back iron 700 shown in FIG. 8 is made upof a number of adjacent back iron laminations 710 that have the sameorientation. The next row of keying structures are similarly made up ofa number of adjacent back iron laminations 710 that have the sameorientation with respect to each other, but have a different orientationto the back iron laminations on the inner axial portion of the laminatedback iron. The next row of keying structures has the same orientation asthe inner axial portion of the inner surface of the laminated back iron.

Having a back iron lamination 710 that has symmetrically repeatingkeying elements provides the advantage of allowing a single back ironlamination design to be used to create a large number of differentkeying structures for a laminated back iron by merely using a suitableorientation for adjacent laminations 710, thereby reducing manufacturingcosts while improving retention of a laminated back iron 700 to a statorheat sink 253.

Once a laminated back iron 700 has been assembled with an appropriatekeying structure formed on the inner surface, the stator heat sink 253is cast on to the inner surface of the laminated back iron 700. Thecasting process allows the material that forms the heat sink 253, inmolten form, to solidify on the inner radial surface of the laminatedback iron 700, thereby resulting in the outer radial surface of thestator heat sink 253, which interfaces with the inner radial surface ofthe laminated back iron, to have complementary keying features to thekeying features formed on the inner radial surface of the laminated backiron 700. Any suitable means may be used to perform the casting process.

The invention claimed is:
 1. A stator for an electric motor, the stator comprising a back-iron having a first surface and a heat sink, wherein the first surface of the back-iron includes a plurality of keying features arranged to retain the heat sink to the back-iron, wherein the heat sink is arranged to provide cooling to the back-iron via surface of the heat sink, and wherein the surface of the heat sink has complementary keying features to the plurality of keying features on the first surface that are formed by casting the heat sink on the first surface of the back-iron, wherein the back-iron is formed from a plurality of laminations of electrical steel, wherein each of the plurality of laminations of electrical steel includes at least one keying feature, and wherein the plurality of laminations that form the back-iron are positioned at rotationally different angles with respect to each other, thereby forming keying features at different rotational positions along the axis of the back-iron.
 2. The stator according to claim 1, wherein the casting of the heat sink onto the first surface of the back-iron includes allowing the heat sink material in molten form to solidify on the first surface of the back-iron.
 3. The stator according to claim 1, wherein the back-iron includes a plurality of stator teeth formed or mounted on a second surface of the back-iron.
 4. The stator according to claim 1, wherein the first surface of the back-iron is a circumferential inner surface of the back-iron formed from the circumferential inner surface of the plurality of laminations of electrical steel.
 5. The stator according to claim 4, wherein each of the plurality of laminations is substantially identical.
 6. The stator according to claim 1, wherein the at least one keying feature of the plurality of laminations of electrical steel are arranged to extend radially inwards from the first circumferential surface of the laminations.
 7. A method of manufacturing a stator, the method comprising: casting a heat sink onto a first surface of a back-iron, wherein the first surface of the back-iron includes a plurality of keying features arranged to retain the heat sink to the back-iron; forming the back-iron from a plurality of laminations of electrical steel, wherein each of the plurality of laminations of electrical steel includes at least one keying feature; and positioning the plurality of laminations that form the back-iron at rotationally different angles with respect to each other to allow keying features to be formed at different rotational positions along the axis of the back-iron.
 8. The method according to claim 7, wherein the step of casting of the heat sink onto the first surface of the back-iron includes allowing the heat sink material in molten form to solidify on the first surface of the back-iron. 